A diffractive optical element is an optical element that is made having a slit-like or groove-like grating structure with several hundred or so narrow slits or grooves at equal intervals for each minute interval (approximately 1 mm). The diffractive optical element possesses the quality of producing a diffracted light beam in a direction determined by the wavelength of the light and the pitch (interval) of the slits or grooves when light is irradiated onto the diffractive optical element. Such diffractive optical elements are used in a variety of optical systems. For example, most recently, a diffractive optical element that is used as a lens by concentrating diffracted light of a specified degree to a point, or the like, has been known.
Among such diffractive optical elements, diffractive optical elements known as multilayer-type diffractive optical elements have been proposed in recent years. A diffractive optical element of this type is rendered by stacking a plurality of diffractive optical elements having faces formed with a saw shape and possesses the characteristic of retaining a high diffraction efficiency substantially in the full range of the desired wavelength range (visible light range, for example), that is, of having a favorable wavelength characteristic.
Generally, a so-called bonded multilayer-type diffractive optical element constituted by diffractive device elements of two types that are mutually different materials that is bonded via the same diffraction grating groove, for example, is known as a multilayer-type diffractive optical element (See Japanese Patent Application Laid Open No. H11-271513, for example).
Further, as shown in FIG. 6, as a multilayer-type diffractive optical element of another example, a so-called separate multilayer-type diffractive optical element constituted by a first diffractive device element 110 and a second diffractive device element 120 that are of different materials and in which the facing saw-like faces of the respective diffraction grating elements 110 and 120 are arranged separated with air therebetween is known. Because height d1 of the diffraction grating groove of the first diffractive device element 110 and height d2 of the diffraction grating groove of the second diffractive device element 120 in the separate multilayer-type diffractive optical element are different, molding using dies for forming the respective diffraction grating grooves is performed. In the case of such a separate multilayer-type diffractive optical element, the height d1 of the diffraction grating groove of the first diffractive device element 110 is determined as a predetermined value and the height d2 of the diffraction grating groove of the second diffractive device element 120 is determined as another predetermined value to satisfy the achromatic condition for the two specified wavelengths. As a result, the diffraction efficiency of the two specified wavelengths is 1.0 and a high diffraction efficiency can also be obtained at other wavelengths. Here, in the case of a transparent-type diffraction grating, the diffraction efficiency is the ratio η(=I1/I0) between the intensity I0 of the incident light and the intensity I1 of first-order diffracted light.
Further, in the case of the bonded multilayer-type diffractive optical element (of Japanese Patent Application Laid Open No. H11-271513), glass is usually used as the material for both the diffraction grating elements constituting the diffractive optical elements, which are made by means of molding using a die in which the diffraction grating grooves are formed, that is, so-called glass-mold molding. However, due to the fabrication time, glass-mold molding has been faced by a multiplicity of technical difficulties such as those of the die molding method, and a glass fabrication method that satisfies requirements.
Further, height d1 of the diffraction grating groove of the first diffractive device element 110 and height d2 of the diffraction grating groove of the second diffractive device element 120 in the separate multilayer-type diffractive optical element (shown in FIG. 6) are different and, because a plurality of dies are required and the diffraction grating elements 110 and 120 must be fabricated separately by means of the same procedure (by using the dies), work is required. In addition, because the two diffractive device elements 110 and 120 must ultimately be fabricated accurately, the diffractive device elements 110 and 120 are extremely hard to make.